Bypass turbofan engines produce two exhaust flows, a core engine flow and a fan flow. The core engine flow is discharged from a core flow nozzle after passing through a core engine. The fan flow passes through an annular passageway formed by a nacelle surrounding the core engine and fan duct. The cross-sectional areas through which these two flows pass may be optimized for specific operating conditions, such as take-off, climb, cruise, etc. Variable area fan nozzles that include movable petals are used to enlarge or reduce the nozzle area of the fan flow, and thereby, decrease or increase the exhaust velocity, respectively. Different forces may act on the petals to move them from their predetermined positions and deviate from a predetermined nozzle area.